The present invention relates to a pre-fabricated building unit which may be used singly, with wall cladding and other associated parts, but is more commonly used in association with other similar units, to constitute the main part of a building such as a house, block of apartments, school, hospital or the like. Many forms of pre-fabricated units of this general type have been made, but have been fairly expensive to manufacture and difficult to transport because of their weight and bulk. Also, they have not generally lent themselves to much flexibility of design, their structural features often greatly limiting the variations in external appearance and interior arrangement of buildings assembled with such units. The provision of adequate ducting for service lines and conduits has also presented a difficulty in many such units hitherto made.